Method and apparatus for accessing an uplink random access channel in a singular carrier frequency division multiple access system

ABSTRACT

A method and apparatus for accessing a contention-based uplink random access channel (RACH) in a single carrier frequency division multiple access (SC-FDMA) system are disclosed. A wireless transmit/receive unit (WTRU) randomly selects a RACH subchannel and a signature among a plurality of available RACH subchannels and signatures. The WTRU transmits a preamble using the selected signature via the selected RACH subchannel at a predetermined or computed transmission power. A base station monitors the RACH to detect the preamble and sends an acquisition indicator (AI) to the WTRU when a signature is detected on the RACH. When receiving a positive acknowledgement, the WTRU sends a message part to the base station. If receiving a negative acknowledgement or no response, the WTRU retransmits the preamble.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/507,712 filed Aug. 21, 2006, which claims the benefit of U.S.Provisional Application Ser. No. 60/710,599, filed Aug. 23, 2005, thecontents of which are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention is related to wireless communication systems. Moreparticularly, the present invention is related to a method and apparatusfor accessing a contention-based uplink random access channel (RACH) ina single carrier frequency division multiple access (SC-FDMA) system.

BACKGROUND

The third generation partnership project (3GPP) and 3GPP2 are currentlyconsidering a long term evolution (LTE) of the universal mobiletelecommunication system (UMTS) terrestrial radio access (UTRA).Currently, SC-FDMA has been adopted for the uplink air interface of theevolved UTRA.

In an SC-FDMA system, a plurality of orthogonal subcarriers aretransmitted simultaneously. The subcarriers are divided into a pluralityof subcarrier blocks, (also known as resource blocks (RBs)). A block ofsubcarriers is a basic resource unit in an SC-FDMA system. Thesubcarrier block may be either a localized subcarrier block or adistributed subcarrier block. The localized subcarrier block is a set ofconsecutive subcarriers and the distributed subcarrier block is a set ofequally spaced non-consecutive subcarriers.

FIG. 1 illustrates two localized subcarrier blocks, each comprising fourconsecutive subcarriers. The localized subcarrier block is a basicscheduling unit for uplink transmissions in a localized-mode SC-FDMAsystem. FIG. 2 illustrates two distributed subcarrier blocks. In thisexample, the distributed subcarrier block 1 includes subcarriers 1, 5and 9, and the distributed subcarrier block 2 includes subcarriers 3, 7and 11. The distributed subcarrier block is a basic scheduling unit foruplink transmissions in a distributed-mode SC-FDMA system. Depending ona data rate or a buffer status, a Node-B assigns at least one subcarrierblock for uplink transmissions for a wireless transmit/receive unit(WTRU).

When a WTRU transitions from an idle mode to a connected mode, the WTRUneeds to communicate with a base station (or a network) using a RACH,which is a contention-based channel. The RACH transmissions of the WTRUhave two parts: a preamble part and a message part. In a conventionalwideband code division multiple access (WCDMA) system, (up to Release6), a transmit power ramping up scheme is used for accessing the RACH.The WTRU starts transmission of a preamble to a base station with a verylow (or minimum) initial transmit power level. If the preamble issuccessfully decoded by the base station, the base station sends apositive acknowledgement (ACK) to the WTRU via an acquisition indicatorchannel (AICH). If the base station fails to decode the preamble, thebase station sends a negative acknowledgement (NACK). When the WTRUreceives a NACK or no response, the WTRU retransmits the preamble whileramping up the transmit power level in subsequent transmission timeintervals (TTIs).

This power ramp up process which starts with a low or minimum powercauses an extra delay for uplink random access which is undesirable.

SUMMARY

The present invention is related to a method and apparatus for accessinga contention-based uplink RACH in an SC-FDMA system. A WTRU randomlyselects a RACH subchannel and a signature among a plurality of availableRACH subchannels and signatures. The WTRU transmits a preamble using theselected signature via the selected RACH subchannel at a predeterminedtransmission power. A base station monitors the RACH subchannels todetect the preamble. The base station sends an acquisition indicator(AI) to the WTRU when a signature is detected on the RACH. When the WTRUreceives an ACK, the WTRU sends a random access message to the basestation. If the WTRU receives a NACK or no response, the WTRUretransmits the preamble. The base station may send a power adjustmentfor the message part and/or timing and frequency correction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional localized subcarrier block for SC-FDMA.

FIG. 2 shows a conventional distributed subcarrier block for SC-FDMA.

FIGS. 3A and 3B are a flow diagram of a process for accessing acontention-based RACH in an SC-FDMA system in accordance with thepresent invention.

FIG. 4 is a flow diagram of a process for processing the preamble in abase station in accordance with the present invention.

FIG. 5 is a block diagram of a WTRU which implements the process of FIG.3.

FIG. 6 is a block diagram of a base station which implements the processof FIG. 4.

DETAILED DESCRIPTION

When referred to hereafter, the terminology “WTRU” includes but is notlimited to a user equipment (UE), a mobile station (STA), a fixed ormobile subscriber unit, a pager, or any other type of device capable ofoperating in a wireless environment. When referred to hereafter, theterminology “base station” includes but is not limited to a Node-B, asite controller, an access point (AP) or any other type of interfacingdevice in a wireless environment.

The features of the present invention may be incorporated into anintegrated circuit (IC) or be configured in a circuit comprising amultitude of interconnecting components.

FIGS. 3A and 3B are a flow diagram of a process 300 for accessing acontention-based RACH in an SC-FDMA system in accordance with thepresent invention. After performing a cell search successfully, a WTRUobtains RACH control parameters (step 302). The RACH control parametersinclude at least one of, but are not limited to:

1) Predetermined transmit power for the preamble (optional) or uplinkinterference level at the Node B, which helps the WTRU to determine thetransmit power of the preamble;

2) Persistence level of transmission on the RACH;

3) Preamble scrambling code;

4) Message length (optional) in time, frequency, or both;

5) AICH transmission timing parameter;

6) A set of available signatures and a set of available RACH subchannelsfor each of a plurality of access service classes (ASCs);

7) Maximum preamble retransmission limit;

8) Power offset P_(p-m) (optional), measured in dB, between the power ofthe control part of the random access message and the power of the restof the random access message;

9) A set of transport format parameters, including a power offsetbetween the data part and the control part of the random access messagefor each transport format; and

10) A one-to-one mapping relation of the time and frequency locationsbetween the RACH and the AICH.

The RACH may be defined by at least one subcarrier, (or at least onesubcarrier block), over at least one time slot. Alternatively, the RACHmay be defined by at least one subcarrier, (or at least one subcarrierblock), over at least one time slot with at least one spreading code. Ifthe RACH is defined with several subcarriers, the subcarriers may beeither consecutive or equally-spaced. Similarly, if the RACH is definedwith several subcarrier blocks, the subcarrier blocks may be localizedsubcarrier blocks or distributed subcarrier blocks. Consecutivesubcarriers and localized subcarrier blocks are preferred overequally-spaced subcarriers and distributed subcarrier blocks because ofless ambiguity in timing detection at the receiver, (e.g., the Node B).

When it is determined at step 304 that there is data to be transmitted,the WTRU selects an ASC from a set of available ASCs (step 306). EachASC is associated with an identifier i of RACH subchannel set and apersistence value P_(i).

A preamble retransmission counter is set to zero (step 308). Thepreamble retransmission counter is then incremented by one beforeinitiating a transmission of a preamble (step 310). It is determinedwhether the preamble retransmission counter exceeds the maximum preambleretransmission limit (step 312). If the retransmission counter exceedsthe maximum preamble retransmission limit, it is indicated to a higherlayer that the maximum preamble retransmission limit has been reached(step 314), and the process 300 ends.

If the retransmission counter does not exceed the maximum preambleretransmission limit, the WTRU checks whether any new RACH controlparameters have been received, and if so, the RACH control parametersare updated with the latest set of RACH control parameters (step 316).

The WTRU then performs a persistence check to determine whether it isallowed to transmit a preamble based on the persistence check (step318). Based on the persistence value P_(i), the WTRU determines whetherto start the preamble transmission procedure in a current random accessinterval. The duration of the random access interval is a designparameter, and may be a single TTI, multiple TTIs or a fraction of aTTI. If the transmission of the preamble is not allowed based on thepersistence check, the WTRU waits for the next random access interval toperform another persistence check in the next random access interval(step 320). The persistency check is repeated until transmission ispermitted. If the transmission of the preamble is allowed based on thepersistence check, the random access procedure is initiated (steps322-328).

The WTRU randomly selects a RACH subchannel among a plurality ofavailable RACH subchannels within the selected ASC (step 322). The WTRUrandomly selects a signature from a set of available signatures withinthe selected ASC (step 324). The random functions for selecting the RACHsubchannel and the signature shall be such that each of the allowedselections is chosen with an equal probability.

The transmission power level for the preamble is set to thepredetermined transmit power value for the preamble (step 326).Alternatively, the transmission power level for the preamble may becomputed using open loop power control and interference information senton a broadcast channel (BCH) from the cell (optional). The predeterminedvalue may be set large enough to ensure that the signal-to-noise ratio(SNR) at the base station meets the predefined threshold in order forthe base station to successfully decode the preamble. Due to the SC-FDMAstructure, the large transmit power of the preamble is limited to thesubcarrier(s), (or subcarrier block(s)), used by the RACH only and doesnot affect other subcarriers or subcarrier blocks in the same cell. In aconventional WCDMA system, the initial transmit power of the preamble isset to a very low level and incrementally ramped up each time thepreamble is retransmitted. This causes a significant delay until thepreamble is detected by the base station. In contrast, in accordancewith the present invention, since the preamble is transmitted at asufficiently high transmission power level, and the RACH subchannel andsignature are selected randomly, such delay is eliminated or reduced.

The WTRU then transmits a preamble using the selected signature via theselected RACH subchannel at the predetermined or computed power level(step 328). After transmitting the preamble, the WTRU monitors an AICHto detect an AI sent by the base station in response to the preamble(step 330). The AICH is a fixed rate physical channel used to carry AIs.The AICH may be spread over several subcarriers to have frequencydiversity and make it more reliable. An AICH may be multiplexed with thedownlink shared control channel. An AI corresponds to a signature on theRACH. There is unique and fixed one-to-one mapping relation of the timeand frequency locations between the RACH and the AICH. With thesignature and the fixed one-to-one mapping relation, the WTRU determineswhich AI is a response to its random access.

If no AI is detected on the AICH, the WTRU waits until the next uplinkRACH sub-channel(s) is available in time domain (step 332), and theprocess 300 returns to step 310 to retransmit the preamble. During theretransmission of the preamble, the transmit power level of the preamblemay or may not be ramped up.

If it is determined at step 330 that a NACK is detected on the AICH, theWTRU waits until the next random access interval (step 334). The WTRUthen sets a backoff timer and waits for the expiration of the backofftimer (step 336). The backoff timer is preferably set to an integermultiple of 10 ms, which is randomly selected between minimum andmaximum backoff periods. The minimum and maximum backoff periods may beset equal when a fixed delay is desired. The minimum and maximum backoffperiods may be set to zero when no delay other than the one due topersistency is desired. After expiration of the backoff timer, theprocess 300 returns to step 310 to retransmit the preamble. During theretransmission of the preamble, the preamble transmission power levelmay or may not be ramped up.

If it is determined at step 330 that an ACK is detected on the AICH, theWTRU transmits a message part to the base station (step 338). Themessage part contains information that a user wants to send to the basestation. The information in the message part may include at least oneof, but not limited to:

-   -   1) Scheduling information, such as WTRU identity, data (traffic)        type, data size, quality of service (QoS) information, and WTRU        transmission power;    -   2) Small amount of traffic data (optional);    -   3) Layer 3 control message;    -   4) Uplink pilot signals; and    -   5) Transport format indicator (TFI) of the transmitted message.

In transmitting the message part, the WTRU may adjust the transmit powerof the message part and timing and frequency according to a poweradjustment and a timing and frequency correction, respectively, whichare generated by the base station, which will be explained withreference to FIG. 4 hereinafter. The message part is transmitted Nuplink access slots after the uplink access slot of the last transmittedpreamble depending on the AICH transmission timing parameter.Transmission power of the control part of the message part should beP_(p-m) dB higher than the transmit power of the last transmittedpreamble. Both N and P_(p-m) are design parameters.

FIG. 4 is a flow diagram of a process 400 for processing the preamble ina base station in accordance with the present invention. The basestation monitors RACH subchannels to detect the preamble (step 402). Thebase station determines whether there is a preamble transmission fromother WTRUs on the same RACH subchannel (step 404).

If there is no preamble transmission from other WTRUs on the RACHsubchannel used by the WTRU, the received SNR at the base station islikely to be high enough to allow the base station to successfullydecode the preamble. After successfully decoding the preamble, the basestation sends an ACK together with the signature of the WTRU back to theWTRU (step 406). Bit-wise multiplication of the ACK with the signaturemay be performed as in the conventional WCDMA system.

The base station may optionally compute a timing and frequencycorrection and transmits them to the WTRU (step 408). Optionally, apower adjustment may also be computed and signaled to the WTRU. The basestation computes the difference between the received SNR and the SNRthreshold that is required for successful decoding to compute the poweradjustment for the WTRU, (i.e., transmit power reduction for thetransmission of the subsequent message part of the WTRU). The poweradjustment P_(adjust) is preferably computed as follows:P _(adjust)=max(SNR_(received)−SNR_(required)−Margin,0);  Equation (1)where Margin is a design parameter. All the parameters in Equation (1)are in the units of dB. The power adjustment may be implicitly carriedin resource allocation information in Node B's response to the preamble.

Because SC-FDMA is more sensitive to the timing and frequencysynchronization errors than a conventional WCDMA system, the basestation may process the preamble and derive the timing and frequencycorrection for the WTRU, and transmit them to the WTRU along with theAI.

If it is determined at step 404 that there is at least one preambletransmitted by other WTRUs on the same RACH subchannel, it is furtherdetermined whether there is a preamble transmitted using the samesignature (step 410). If there is at least one preamble transmittedusing the same signature, a collision occurs and the base station sendsa NACK to the WTRUs involved in the collision, (i.e., sends a NACK forthe signature) (step 412). The base station may transmit the NACK withthe signature. For example, bit-wise multiplication of the NACK with thesignature may be performed as in the conventional WCDMA system.

If it is determined at step 410 that there is no preamble transmittedusing the same signature, the received SNR may or may not meet therequired SNR for successful decoding due to the near far problem orinterference caused by cross-correlation between signatures. The basestation generates an ACK for the WTRU whose received SNR meets therequired SNR, (i.e., ACK for the signature used by the WTRU), and doesnot generate either an ACK or a NACK for the WTRU whose received SNRdoes not meet the required SNR (step 414). The base station may computethe power adjustment and the timing and frequency correction for theWTRU whose received SNR meets the required SNR.

FIG. 5 is a block diagram of a WTRU 500 which implements the process ofFIG. 3. The WTRU 500 includes a RACH processor 502 and a transmitter504. The RACH processor 502 is configured to randomly select a RACHsubchannel among a plurality of available RACH subchannels and asignature among a plurality of available signatures. The transmitter 504is configured to transmit a preamble using the selected signature viathe selected RACH subchannel at a predetermined or computed transmissionpower level. The WTRU 500 may include a retransmission counter 506. Theretransmission counter 506 is for tracking the number of retransmissionsof the preamble. The retransmission counter 506 is initialized attransmission of a new preamble and incremented each time the preamble isretransmitted. The transmitter 504 transmits the preamble only if theretransmission counter 506 does not exceed a retransmission limit.

FIG. 6 is a block diagram of a base station 600 which implements theprocess of FIG. 4. The base station 600 includes a preamble detector 602and an AICH processor 604. The preamble detector 602 is configured todetect a preamble transmitted by a WTRU on a RACH. The AICH processor604 is configured to send an AI to the WTRU when a preamble is detectedon the RACH. The base station 600 may also include a transmit powercontroller 606 and/or a timing and frequency controller 608. Thepreamble detector 602 determines whether there is a preamble transmittedby another WTRU on the selected RACH subchannel and the AICH processor604 sends an ACK if there is no preamble transmitted by another WTRU onthe selected RACH subchannel.

The transmit power controller 606 is configured to compute a poweradjustment based on a received power level of the preamble. The AICHprocessor 604 sends the power adjustment to the WTRU along with the AIto adjust the transmit power level of the message part. The timing andfrequency controller 608 is configured to compute a timing and frequencycorrection based on the preamble. The AICH processor 604 sends thetiming and frequency correction to the WTRU along with the AI to adjusttiming and frequency.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention.

What is claimed is:
 1. A wireless transmit/receive unit (WTRU)comprising: a processor and a transmitter configured to: transmit arandom access preamble, using single carrier frequency division multipleaccess (SC-FDMA), based on obtained control information; monitor for areceived response to the random access preamble, which includes a timingadvance, a power command and a resource allocation; and transmit arandom access message using the SC-FDMA, having a timing derived atleast from the timing advance, a transmission power level derived atleast from the power command and utilizing resources derived from atleast the resource allocation.
 2. The WTRU of claim 1 wherein theresource allocation indicates one or more resource blocks to transmitthe random access message.
 3. The WTRU of claim 1 wherein the one ormore resource blocks indicate a plurality of subcarriers to transmit therandom access message.
 4. The WTRU of claim 3 wherein the indicatedsubcarriers are consecutive subcarriers or distributed subcarriers. 5.The WTRU of claim 1 wherein the random access preamble is derived from asequence, the sequence being indicated by the received response.
 6. TheWTRU of claim 1 wherein the transmitter is configured to transmit therandom access preamble including pilot signals.
 7. The WTRU of claim 1wherein the transmitter is configured to transmit the random accesspreamble having a duration which is variable.
 8. The WTRU of claim 1wherein the transmitter is configured to transmit a subsequent randomaccess preamble at an increased power level relative to a previouslytransmitted random access preamble.
 9. The WTRU of claim 1, configuredto: perform a cell search; and obtain the control informationthereafter.
 10. A method implemented by a wireless transmit/receive unit(WTRU), the method comprising: transmitting a random access preamble,using single carrier frequency division multiple access (SC-FDMA), basedon obtained control information; monitoring for a received response tothe random access preamble, which includes a timing advance, a powercommand and a resource allocation; and transmitting a random accessmessage, using the SC-FDMA, having a timing derived at least from thetiming advance, a transmission power level derived at least from thepower command and utilizing resources derived from at least the resourceallocation.
 11. The method of claim 10 further comprising indicating,via the resource allocation, one or more resource blocks to transmit therandom access message.
 12. The method of claim 11 further comprising:indicating, via the one or more resource blocks, a plurality ofconsecutive or distributed subcarriers to transmit the random accessmessage.
 13. The method of claim 10, further comprising deriving therandom access preamble from at least a sequence indicated by thereceived response.
 14. The method of claim 10, further comprisingincluding pilot signals in the random access message.
 15. The method ofclaim 10, further comprising setting a duration of the random accesspreamble, wherein the transmitting of the random access preambleincludes transmitting the random access preamble of the set duration.16. The method of claim 10, further comprising: performing a cellsearch; and obtaining the control information thereafter.